Medication monitoring measures the level of a medication in the blood to make sure the dosage of medication is correct. Sometimes the amount of medication that helps (therapeutic level) is very close to the amount that can cause harm (toxic level). Determining the correct dosage of a medication for each person can be difficult, since the dose needed to produce a therapeutic level in the blood varies from person to person. Factors that affect medication levels in the blood include age, weight, activity level, the speed at which the body breaks down the medication, how the medication was taken (by mouth, patch, or shot), and other medications a person is using.

Medication monitoring is important for people whose bodies do not break down (metabolize) medications effectively because of poor function in the liver or kidneys. Also, some medications can damage an organ (such as the liver or kidneys) or cause other problems at the dose used for treatment. If you are taking such a medication, its amount in your blood may need to be checked regularly to ensure your dosage is at a safe and effective level.

Medication monitoring can also be useful for infants, older adults, people with heart failure or changes in heartbeat (arrhythmia), or people taking two or more medications that may interfere with each other.

Medication blood levels often depend on when the last dose was taken. The highest, or peak, level usually occurs 30 to 60 minutes after taking the medication. The lowest, or trough, level occurs right before taking the next dose.

Monitoring can be done for nearly any type of medication, but it is most often done for:

• Certain antibiotics, such as amikacin (Amikacin), gentamicin, or tobramycin (Nebcin). These antibiotics may cause hearing loss or kidney damage if their level in the blood gets too high.
• Medications to treat heart problems. These include digoxin (Lanoxin), quinidine (Cardioquin, Quinidex, Quinaglute, Quin-Release), or procainamide.
• Medications to treat seizure disorders (epilepsy). These include phenytoin (such as Dilantin), carbamazepine (such as Tegretol), and valproic acid (Depacon, Depakote, Depakene).
• Asthma medications, such as theophylline.
• Barbiturates, such as phenobarbital.
• Medications to treat bipolar disorder, such as lithium (Eskalith).
• High doses of aspirin or similar medications (salicylates).
• Medications used to prevent the body from rejecting an organ transplant, such as cyclosporine (Neoral, Sandimmune, SangCya).

A prothrombin time (PT) test is used to monitor the effects of the blood-thinning medication warfarin (Coumadin).

Why It Is Done

Medication monitoring may be done to:

• Confirm you are receiving an effective dose of medication.
• Determine whether you are receiving too much medication.
• Ensure that you are taking a medication properly.

How To Prepare

It is important to know exactly when you took the last dose of the medication for which you are being tested. Testing is usually done right before you take another dose. Ask your health professional for instructions about the timing of the test.

Tell your health professional about all the prescription and nonprescription medications you are taking and any drugs (such as alcohol, marijuana, or cocaine) you are using. Sometimes these can interfere with testing.